In the past, various catalysts have been employed for the polymerization of olefins, said catalysts comprising those known in the art as Ziegler-Natta catalysts. These catalysts typically consist of titanium chloride which has been activated with an aluminum alkyl, said catalysts operating in the form of a sludge or slurry. For example, titanium tetrachloride may be used in conjunction with an organic compound such as triethylaluminum or other organoaluminum compounds. The titanium tetrachloride is reduced to a lower valent titanium chloride by the organoaluminum compound. A variation of this type of catalyst comprises impregnation of alumina with a titanium tetrahalide followed by reduction with hydrogen at elevated temperatures or by reduction effected by contact with a solution or dispersion of a reducing agent such as an alkali or alkaline earth metal halide, etc. However, these catalysts are useful in polymerizing olefins to form solid polymers, and specifically, high molecular weight solid polymers in which the molecular weight will range from 300 to 100,000. Catalysts of this type is preparing solid polymers usually employ aromatic hydrocarbons such as benzene, toluene, xylene as suitable diluents. The use of a diluent such as an aromatic hydrocarbon may not be employed when utilizing the heterogeneous oligomerization catalyst which is used in the present process inasmuch as the aromatic hydrocarbon, if present, would enter into the reaction in which parts or all of the olefin would act as an alkylating agent rather than as a monomer in the polymerization reaction.
Other catalysts which have been used for polymerizing olefins also comprise titanium tetrahalides composited on a solid support such as an alumina, the finished composite having been prepared in various ways such as by activating alumina at an elevated temperature followed by treating the activated base with a mixture of an inert gas and titanium tetrachloride or by treating an alumina gel with fluotitanic acid. As was previously stated, these catalysts may be employed to polymerize olefins to heavier hydrocarbons, that is, solid polymers and, in addition, may also be used to alkylate paraffins with olefins, usually at temperatures which are relatively high in nature.
Prior patents have shown various processes involving the polymerization of olefins. For example, U.S. Pat. No. 3,109,041 shows a process for polymerizing isobutylene having a molecular weight of from about 300 to about 10,000, the process involving two stages. In the first stage, butylene is reacted in admixture with an inert solvent in contact with a Friedel Crafts catalyst in the absence of a support. The second stage involves the reaction in the presence of a fixed bed supported Friedel Crafts catalyst with the purpose of obtaining total polymerization. British Pat. No. 734,264 utilizes a catalyst which is chemisorbed on a binary or tertiary gel such as silica-alumina, silica-magnesia, silica-zirconia, etc., silica-aluminamagnesia, etc., the objective of the process being the production of gasoline.
In addition to these patents, two other U.S. patents also teach a process for oligomerizing olefinic compounds. U.S. Pat. No. 4,108,920 utilizes, as a catalyst for the reaction, a compound which has been prepared by heating a metal oxide which possesses surface hydroxyl groups with hydrogen and nitrogen at an elevated temperature which may range from about 350.degree. to about 550.degree. C. and thereafter impregnating the heated metal oxide with a solution of titanium tetrafluoride. The impregnated oxide is then cold-rolled, followed by steam drying and further drying the component at an elevated temperature of from about 200.degree. to about 600.degree. C. in an inert atmosphere. Likewise, U.S. Pat. No. 4,110,410 also discloses a process for the oligomerization of olefins using a catalyst which has been prepared by heating a metal oxide again possessing surface hydroxyl groups at a temperature in the range of from about 400.degree. to about 600.degree. C., contacting the metal oxide with a titanium tetrachloride vapor in a series of steps at progressively higher temperatures whereby the titanium tetrachloride is composited on the metal oxide. The impregnated metal oxide is then heated in contact with hydrogen at elevated temperatures ranging from about 300.degree. to about 700.degree. C. to reduce the titanium to a valence state of less than +4 and thereafter using this catalyst to oligomerize olefins. U.S. Pat. Nos. 4,048,108 and 4,048,109 disclose methods for preparing the catalysts which are utilized in the aforementioned two U.S. patents. However, the process using these catalysts results in a product mix which is heavy in highly branched-chain products. As will hereinafter be shown in greater detail, by utilizing the catalysts of the present invention, it is possible to obtain selective oligomers of olefinic hydrocarbons, the oligomers being selectively less branched compounds.